It is well known that certain types of light trailers have a tendency to sway or weave from side to side instead of tracking stably behind the towing vehicle. When the trailer is towed by a four-wheel vehicle such as an automobile or a truck, the towing vehicle usually has sufficient mass and stability to resist being turned by the trailer. But if the towing vehicle is a motorcycle, swaying or weaving of the trailer obviously creates a very dangerous condition.
In general, assuming that a two-wheel trailer is properly hitched and that its wheels are tracking properly, it is usually caused to sway or weave by wind gusts, by bumps in the pavement, or by a combination of these. A motorcyclist is naturally well aware of wind conditions and, if nothing else, can avoid towing a trailer in gusty weather. But pavement irregularities may be encountered at any time, and they are usually unforeseen and unexpected. Thus an important safety requirement for a motorcycle-towed trailer is that it tow straight and true on bumpy surfaces as well as on perfectly smooth pavement.
It has been recognized for some time that a two-wheel trailer has a more stable ride, with less weaving and side sway, if its wheels are independently suspended so that each can make vertical shock absorbing movements relative to the frame without influencing the other wheel or being influenced by it. This is pointed out in U.S. Pat. No. 2,549,320, issued to M. E. Makin in 1950, which disclosed a light two-wheel trailer wherein the wheels had a semi-independent suspension that was said to eliminate "any tilting or lateral swaying" of the trailer bed and to minimize "the danger of upsetting a vehicle caused by an unevenly distributed or shifting load." However, the two wheels of the trailer disclosed by Makin were mounted on the opposite ends of a single rigid axle that extended across the trailer, so that both wheels changed their orientations when one of them rose or fell in passing over a bump.
Various arrangements for independent wheel suspensions have been proposed from time to time, as for example in U.S Pat. No. 3,201,142, issued to M. Dangauthier in 1965 and U.S. Pat. No. 2,905,575, issued to H. Locker in 1959, but most of these have been intended for relatively large and heavy vehicles and have been inherently unsuitable for adaptation to a motorcycle trailer, which has stringent requirements not imposed by larger vehicles.
To avoid braking and acceleration problems, a motorcycle trailer should weigh as little as possible, but it must nevertheless be sturdy. For the best combination of strength and lightness, the body of the trailer should be mounted on a bed or frame to which the wheels are connected. However, the frame and the wheel suspensions must be so arranged as not to require sacrifice of load capacity of the body while nevertheless supporting the body at a level no higher than is adequate for road clearance, to keep the center of gravity low. The frame and the wheel suspensions should also be so arranged that the body can be reasonably well streamlined, to minimize wind forces that could create undue drag on the motorcycle as well as causing swaying or weaving of the trailer.
It is a premise of this invention that the tracking stability and freedom from side sway that are desired in a motorcycle trailer are best achieved by providing the trailer with independent wheel suspensions of the type comprising a trailing arm for each wheel, wherein the wheel axle is rigidly connected to a rear end of the trailing arm and the front end of the trailing arm has a pivot connection to the trailer frame that enables the wheel to swing up and down relative to the frame. A shock absorbing means must of course be connected between the trailing arm and the relatively fixed part of the trailer frame. The arrangement of the trailing arm, its mounting, and the shock absorbing means must be such that these elements are compatible with a body that meets the requirements mentioned above.
The wheels of a motorcycle trailer are necessarily small, so that they are affected by every small irregularity in pavement and are therefore constantly moving up and down in their shock absorbing motion relative to the frame. This constant action imposes stringent requirements upon the pivot connections between the frame and the trailing arms, which must be capable of a long useful life with a minimum of attention.
One other consideration is worthy of mention. The cost of transporting a motorcycle trailer from the manufacturer to the ultimate user is necessarily included in the price that the user pays for it. Transportation costs can be reduced if the trailer can be shipped in a partially disassembled condition that materially reduces its bulk, but such a gain should not be realized at the cost of requiring complicated or time-consuming assembly of the trailer at its destination or sacrifice of sturdiness or dependability.